Crude oil serves as the dominant source of motor fuel and feedstock for the petrochemical industry. The quality, or economic value, of crude oil is determined based on the cleanliness and lightness of the crude. Cleanliness refers to having a low content of impurities, e.g., sulfurs, metals, asphaltenes. Lightness refers to the concentration of light and middle distillates. Middle distillate is generally defined as diesel-range hydrocarbons which have a boiling point between 175° C. and 370° C., depending on the specification of the diesel fuel of the country and refinery. Light distillate is generally defined as naptha-range hydrocarbons having lower boiling points than those of middle distillate. Upgrading processes are used to clean and lighten crude oil and other hydrocarbon streams, by subjecting those streams to various chemical reactions. There are a number of upgrading processes, many of which are designed for certain crude fractions, such as vacuum gas oil or vacuum residue.
Crude oil consists of a mixture of various types of molecules, including alkanes, alkenes, aromatics, and napthenes. Crude oil can be separated, based on boiling points, into gas, naptha, kerosene, diesel, vacuum gas oil (VGO), and vacuum residue (VR). A hydrocarbon solvent, such as n-heptane, can be used to separate crude oil into maltene and asphaltene. Asphaltene, primarily concentrated in vacuum residue, is most commonly thought to be a cluster of aromatic cores, linked by a non-aromatic network. The structure includes heteroatoms, such a sulfur, nitrogen, and oxygen and metals.
The precise compositions of crude oil or upgraded oil streams are unknowable due to the number of components present. Nor can composition be predicted based on the source of the crude oil or the nature of upgrading reactions. A crude oil from one source subjected to an upgrading reaction would have a product composition different from a crude oil from a second source subjected to the same upgrading reaction. Conversely, a crude oil subjected to an upgrading reaction aimed to desulfurize the stream would have a different composition if the same crude oil were subjected to a different desulfurization process. The inability to know the composition of the crude oil has led to the use of other measurements as ways to classify oil. One classification is boiling curve analysis, in which the temperature of a definitive volume percentage of a distillate fraction is measured. For example, a T95 is the temperature at which 95% of the distillate in a distillation column is vaporized. While T95 is a concept related to End Boiling Point (EBP), T95 provides a more representative value, because the presence of tiny amounts of large molecules can make EBP very high. The points, i.e., T95, can then be correlated to other industry developed property measures such as specific gravity, molecular weight, viscosity, and the API value.
Upgrading reactions can be used to preferentially generate certain components in the product. One such group of components that is targeted is aromatics. Aromatics are of increasing importance in industrial applications. For example, blending aromatics with gasoline results in a blended gasoline with a high octane rating.
Benzene, toluene, and xylene are three aromatics used in a variety of plastics, synthetic rubbers, and fibers. Collectively referred to as BTX, these aromatics can be preferentially produced by certain upgrading processes such as steam cracking or a fluidized catalytic cracking process or a reforming process. Most olefins, especially ethylene and propylene, are produced by such thermal processes.
In a steam cracking process, naptha is cracked to produce olefins, aromatics, and other hydrocarbons in the absence of a catalyst. The steam serves as a diluent, reducing the concentration of reactant and the generation of solid coke. Steam cracking processes can be designed to preferentially produce olefins. Steam cracking also can produce pyrolysis gasoline which can contain 70% by weight aromatics. Pyrolysis gasoline can be blended in the gasoline pool or subjected to further treatment. Steam cracking produces coke as a byproduct of the process.
Another thermal process is a fluidized catalytic cracking (FCC) process. In an FCC process, heavy hydrocarbons, such as vacuum gas oil and vacuum residue are converted to light hydrocarbons, including olefins, gasoline, and diesel. FCC processes are used in refineries to produce gasoline having a high octane rating and a light cycle oil (LCO) diesel fraction. Heavy cycle oil (HCO) is a blend for fuel oil, because the blend reduces the viscosity of the fuel oil. FCC processes use catalysts, which can be formulated to preferentially produce certain reaction products, but catalysts are expensive and suffer from deactivation, requiring reactivation in a separate regeneration unit to maintain productivity. Additionally, FCC units produce undesirable heavy products such as slurry oil and coke.
In a reforming process, straight run naptha is converted to a gasoline blendstock having a high octane rating due to an increased aromatic content. The aromatic content is increased using catalysts for dehydrocyclization, aromatization, cyclization, and dehydrogenation reactions in the presence of a large external supply of hydrogen. Reforming processes produce hydrogen in addition to hydrocarbon products. In general, the reformate product from a reforming process contains around 50 weight percent of aromatics. Like an FCC process, the catalyst in a reforming process is expensive and prone to deactivation by coking Coking can occur with slight fluctuations in the process or feedstock properties and conditions. Due to the expense of the catalyst, a regeneration unit is required to reactive spent catalyst.
Chemical reactions in the presence of supercritical water, such as supercritical water oxidation and supercritical water hydrolysis processes, are increasingly being explored. Supercritical water serves as a diluent, which reduces inter-radical reaction and the development of coke, and serves as a source of hydrogen. Thus, supercritical water shows promise as a reaction medium due to the ability to upgrade without the requirement of an external supply of hydrogen gas. Additionally, supercritical water can accelerate certain groups of reactions, such as cracking, which reduces reactor size. Supercritical water exhibits high selectivity which reduces coke generation.
Supercritical water is an alternate to the use of catalysts, because many catalysts are not stable under supercritical water conditions.
Upgrading reactions in the presence of supercritical water undergo radical chain reaction. It is expected that many of the above bonds crack immediately after being introduced to a supercritical water reactor due to the thermal energy present in the supercritical water. Carbon-heteroatom bonds, like carbon-sulfur bonds, including thiols and sulfides, carbon-nitrogen bonds, carbon-metal bonds, and weak carbon-carbon bonds are easily broken and generate radicals. The radicals of the broken bonds initiate the radical chain reaction. Radicals on molecules are propagated to other molecules which results in rearrangement of the molecular structure of the molecules to achieve cracking, oligomerization, isomerization, dehydrogenation, cyclization, aromatization, and other reactions. The order and nature of the products of upgrading reactions are sensitive to the operating conditions of the reactor and, therefore, are difficult to predict. Aromatic compounds, especially light aromatics such as benzene, toluene, and xylene (BTX) are self-inhibitors. That is, the delocalization of the radical by the aromatic structure causes a decrease in the concentration of radicals and an increase in the termination step.
The dominant source of BTX in supercritical water reactions is believed to be heavy aromatic compounds having a single aromatic core with an alkane chain. These heavy aromatic compounds are concentrated in asphaltene fractions, such as in vacuum residue. Dehydrocyclization of alkanes and dehydrogenation of napthenes also produce BTX, but in smaller quantities because of the complexity of the reaction network and the presence of competing reaction routes such as cracking.
Once a product stream rich in aromatics is produced, the BTX can be separated by distillation or by solvent extraction. Solvent extraction will not work for stream with low aromatic content, so in some cases concentrating the BTX between 20 vol % and 65 vol % is necessary prior to an extraction unit. Distillation takes advantage of the boiling point differences between hydrocarbon compounds, where the boiling point range of a naptha distillate is between 39° C. and 200° C. Solvent extraction, or liquid-liquid extraction, is a process whereby a solvent is used to separate a compound from a liquid based on its solubility in the solvent as compared to the solubility of the rest of the liquid.